Relative Humidity

Definition

Relative Humidity (RH) is the ratio of the actual partial pressure of water vapour in zone air to the saturation vapour pressure at the prevailing air temperature, expressed as a percentage. It is sensed directly by co-located capacitive polymer hygrometers on the PhytoSense Mesh, paired one-to-one with the canopy-plane temperature probes so that RH and temperature share an air parcel and can be combined into VPD without spatial error.

Why it matters

RH is one half of the transpiration equation. Too low and the canopy desiccates, stomata close, and growth stalls; too high and leaf boundary layers saturate, guttation and condensation appear, and the zone becomes a culture plate for Botrytis and powdery mildew. Together with temperature it yields VPD (VC-MET-102), the variable CHLORA actually controls to. The Directorate watches RH directly because the disease-risk edge (> 85 %) is governed by absolute humidity, not VPD alone.

Formula

A directly measured signal. Zone RH is the trimmed mean of the hygrometer cluster, temperature- compensated at the node:

RH_zone = trimmed_mean( { RH_i + c_i(T)  :  i ∈ cluster } )

  RH_i    raw capacitive reading (%), node i
  c_i(T)  temperature-compensation term from node cal curve

Saturation reference (Tetens), used downstream for VPD:
  e_s(T) = 0.6108 · exp( 17.27·T / (T + 237.3) )   [kPa]
  e_a    = (RH/100) · e_s(T)                        [kPa]

Outlier rejection:
  drop any RH_i where |RH_i − median| > 4 %RH before averaging.
Condensation guard:
  readings pinned at 100 %RH for > 5 min flag probe wet-out.

Inputs

Units & Scale

Reported as percent relative humidity to whole-percent precision. RH is intensive and never summed; facility roll-up is the area-weighted mean with a 24-hour min/max envelope. Confidence degrades to DEGRADED when the chilled-mirror reference disagrees with the capacitive cluster by > 5 %RH, or when any probe pins at saturation (wet-out), which biases capacitive sensors high until dried.

Sampling & Source

• Sampled every 1 minute from the PhytoSense Mesh hygrometer cluster.
• Each RH probe is co-located with a temperature probe so VPD is computed from a single air parcel.
• Stale handling: a probe silent > 3 min is dropped; loss of the full cluster fails the zone over to the dew-point reference with a DEGRADED flag.
• Calibration: quarterly two-point salt-bath check (33 % / 75 %); drift > 2 %RH raises a metrology work order.

Thresholds

Recent Trend

Zone-A canopy-plane relative humidity, last 14 sampling roll-ups (% RH):

Interpretation Guidance

Related Metrics

Related SOPs

• SOP Library — humidity control & disease-prevention procedures.
• Fungal-disease scouting on high-RH CRIT — see Environment SOP set in the SOP Library.
• Alarm escalation & dehumidification interlocks — Monitoring Systems.